can anyone tell me if there are any perticuular laws that regulate the way collection agencies must go about debt settlelment (assuming they're in agreement)?

i was able to get one agency to reduce my debt for a certain amount. I was to pay half of it on the spot, and the other half the following month. They collected the first payment. I was late on the second-but i did pay it.

it turns it out that they "cancelled" the original agreement because of the late payment, but they still collected both payments....AND I STILL OWE.

is this legal? how can i find out what to do without going through the hassles of trying to get legal advice from an attorney...